Implement Light and Dark Theme System

[leo7418]

The application should support both light and dark themes with a seamless switching mechanism and proper theme management system.

Current State

The application currently has:

• Theme system implemented: Complete light and dark theme files (assets/themes/light.txt and assets/themes/dark.txt) with comprehensive styling for all UI components
• Theme loading disabled: Theme loading is commented out (// tgui::Theme::setDefault("assets/themes/Black.txt"); in Application.cpp:26)
• No theme selection mechanism: No way for users to choose between themes
• Hardcoded colors: Manual color adjustments scattered throughout the codebase instead of relying on theme files

Requirements

Core Theme System

• Theme Activation: Enable theme loading and set a default theme on application startup
• Theme Persistence: Theme selection should be saved to application settings and restored on startup
• Dynamic Theme Switching: Users should be able to change themes without restarting the application
• Theme Detection: Support system theme detection (when available on the platform)

Theme Configuration

• Remove Hardcoded Colors: Eliminate all manual color adjustments in the codebase and rely entirely on theme files
• Theme Consistency: Ensure all UI components use colors defined in theme files
• Icon Support: Implement automatic icon switching based on theme (already has black/white variants in assets/images/)

User Interface

• Theme Selector: Add theme selection in the Settings page under "Application Settings"
  • Options: "System", "Light", "Dark"
  • Default to "System" when available, fallback to "Dark"
• Real-time Preview: Theme changes should apply immediately without restart
• Visual Feedback: Show current theme state in the settings UI

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The implementation should follow the existing MVC architecture and leverage the comprehensive theme files already created, focusing on removing hardcoded colors and implementing the theme selection mechanism.